1350-1400;Middle Englishal redyall ready; what orig. meant “completely (all) ready” and modified the subject (The porter all ready was there) was taken adverbially as modifying the predicate (The porter already was there, meaning “from an earlier time”)

Usage note

Although already and all ready are often indistinguishable in speech, the written forms have distinct meanings and uses. The phrase all ready means “entirely ready” or “prepared” (I was all ready to leave on vacation). Already means “previously” (The plane had already left the airport) or “so soon” (Is it lunchtime already?).

c.1300, compound of all + ready (adj.); literally "fully ready." Cf. Norwegian, Danish allerede "already." Colloquial use in U.S. as a terminal emphatic (e.g. enough, already!) is attested from 1903, translating Yiddish shoyn, which is used in same sense. The pattern also is attested in Pennsylvania German and in South African.